The North Sea Mine Barrage, also known as the Northern Barrage, was a large minefield laid easterly from the Orkney Islands to Norway by the United States Navy during World War I. The objective was to inhibit the movement of U-boats from bases in Germany to the Atlantic shipping lanes bringing supplies to the British Isles. The idea of a barrage across the North Sea was first proposed in the summer of 1916 by Admiral Reginald Bacon and was agreed at the Allied Naval Conference on 5 September 1917. A minefield across the North Sea would require mining water 900 feet deep while no previous minefield had been established in more than 300 ft deep. A minefield across the North Sea had been estimated to require 400,000 conventional anchored mines. An antenna mine developed in July 1917 was effective at the assumed maximum submarine depth of 200 ft, and 100,000 of these new Mk 6 mines would be adequate to form the North Sea mine barrage. The U. S. Navy tendered an order for the Mk 6 mines in October 1917 with 80,000,000 ft of steel wire rope required to moor the mines to the seabed. Project spending of $40 million was shared among 140 manufacturing contractors, all mine components other than wire rope, explosives, and detonating circuitry were manufactured by Detroit automobile firms. The objective was to prevent U-boats from operating in the North Atlantic, a similar barrage had already been placed across the English Channel, which had resulted in U-boats diverting north around Scotland. The North Sea Mine Barrage was intended to close this alternative route, the Mk 6 mine was a 34 in diameter steel sphere containing a buoyancy chamber and 300 lb of TNT. Each mine was constructed of two steel hemispheres welded together, a Toxyl bursting charge was cast into the lower hemisphere. For transport, the mine rested atop a box-shaped steel anchor approximately 30 inches square, the anchor box had wheels allowing the mine assembly to be moved along a system of rails aboard the minelayer. The mine was connected to its 800-pound anchor box by a wire rope mooring cable stored on a reel, the sensor locked the cable reel so the falling anchor would pull the buoyant mine below the surface, and the float extended the antenna above the mine. Each mine had two safety features intended to render the mine safe if it detached from its mooring cable. The first was a switch in the detonation circuit closed by hydrostatic pressure. The second was a spring pushing the detonator away from the charge into the buoyancy chamber unless compressed by hydrostatic pressure. The mines were intended to be safe at depths less than 25 ft, each mine contained a dry cell battery with an electrical detonating circuit which could be initiated by any one of five parallel fuzes. Four of the fuzes were conventional horns in the buoyant upper hemisphere of the mine, each horn contained a glass ampule of electrolyte which would connect an open circuit if an ampule was broken by bending the soft metal horn
A Mk 6 mine atop its anchor. Two horn fuzes are visible, but the antenna fuze cannot be seen in this image.
Only the two smallest of the eight steamships converted to lay the barrage remained in commission for conventional minelaying operations. USS Shawmut, shown laying the North Sea mine barrage, sank 23 years later during the attack on Pearl Harbor after being renamed Oglala.
USS Eider (Minesweeper No. 17) (left) in port with submarine chasers alongside during the clearance of the North Sea Mine Barrage in 1919. The leftmost submarine chaser is either SC-254, SC-256 or SC-259 and the others are (left to right) SC-45, SC-356, SC-47, and SC-40.